1. Field of the Invention
The present invention relates to a method of manufacturing a fine T-shaped electrode at low cost and with high throughput without using expensive processes such as electron beam lithography and SOR light exposure.
2. Description of the Related Art
Fine T-shaped electrodes are used as gate electrodes in electric field effect transistors in advanced telecommunications using high frequencies, such as the microwave and millimeter waveband. In this case, it is required that the gate length of the base part in the fine T-shaped electrode is 0.1 μm or less. The overgate part is usually designed to be larger than the base part in the fine T-shaped electrode due to the need to make the gate resistance value as low as possible.
When manufacturing this fine T-shaped electrode, in the related art, an opening which determines the gate length dimension in a lower layer of a laminated resist, and an opening which determines the size of the overgate part in an upper layer of the laminated resist, are generally formed. In this case, in order to obtain a laminated resist in which openings of different size are formed in separate layers, a plurality of resist materials having different sensitivities are required. Further, to use the lift-off method to manufacture the fine T-shaped electrode, an intermediate layer resist which ensures easy lift-off must be provided in the laminated resist. Since there are a large number of electron beam resists having different sensitivities and the gate length of the base part in the fine T-shaped electrode is required to be 0.1 μm or less, the fine T-shaped electrode was usually manufactured using electron beam resists in the laminated resist by high cost electron beam lithography. However, the fine T-shaped electrode could not be manufactured at low cost with high throughput.
In order to solve the problems, a process has been proposed which makes partial use of light exposure with a multilayer resist comprising not only an electron beam resist, but also a photoresist with superior throughput to electron beam lithography, to manufacture the fine T-shaped electrode. For example, in Japanese Patent Application Laid-Open No. 07-153666, the following process is described. As shown in FIG. 23A, an opening 12 is formed in a lower layer resist 11 coated on a substrate 20 by electron beam lithography, an upper resist 14 is coated thereon, and a mixing layer 15 of the upper resist 14 and lower layer resist 11 is thereby formed. By irradiating the upper resist 14 with ultraviolet light 16 to form a cross-sectional T-shaped pattern as shown in FIG. 23B, forming a T-shaped gate electrode as shown in FIG. 23C, and removing the lower layer resist 11 and other resists as shown in FIG. 23D, a fine T-shaped gate electrode is manufactured. Herein, an overgate portion where an overgate part of the T-shaped gate electrode 18 is to be formed, is formed by light exposure, and the gate length of the base part in the T-shaped gate electrode 18 is shortened by forming an opening by electron beam lithography using an electron beam resist as the lower layer resist 11.
However, in order to obtain a gate length of 0.1 μm or less, the opening 12 provided in the lower layer resist 11 had to be formed by electron beam lithography, and there was, therefore, a problem of high cost and low throughput.
On the other hand, JP-A No. 11-307549 describes forming a fine T-shaped electrode by patterning with high throughput and as finely as by EB exposure using an i line stepper. As shown in FIG. 24A, a resist film 23 for EB exposure, a buffer film 24 and a resist film 25 for i line exposure are successively formed on a substrate 20. Subsequently, the resist film 25 for i line exposure and the buffer film 24 is subjected to a patterning by i line exposure to thereby form a first opening 25a. Next, as shown in FIG. 24B, dry etching is performed on the resist film 23 for EB exposure using the resist film 25 for i line exposure as a mask, and the pattern of the i line exposure resist film 25 was transferred to the EB exposure resist film 23 to thereby form a second opening 23a. 
Next, as shown in FIG. 24C, a third resist film 26 which is a chemical amplification type and can form a mixing layer 27 with the resist film 23 for EB exposure is applied all over the resist film 23 for EB exposure. Thereby, the wall surface of the second opening 23a is covered with the mixing layer 27, and the opening width of the second opening 23a is reduced. Next, as shown in FIG. 24D, patterning is performed by i line exposure on the third resist film 26. By forming an electrode and removing the resist film, a T-shaped gate electrode is thus obtained. In this case, as electron beam lithography is not used and only light exposure by the i line stepper is used, low cost is achieved.
However, exposure by the i line stepper must be performed twice, i.e., when the opening of the resist film 25 for i line exposure is formed and when the opening of the third resist film 26 is formed, and this does not provide a sufficiently high throughput. The opening of the resist film 25 for i line exposure is around 0.4 μm and the first opening 23a has similar dimensions, but there is a problem in that when the opening is reduced by the mixing layer 27 from these dimensions to 0.1 μm or less, dimensional control is poor and difficult to perform. Also, since the T-shape is formed by two layers, i.e., the third resist film 26 and the mixing layer 27, there is a problem in that lift-off of the resist film when the fine T-shaped gate electrode of 0.1 μm or less is manufactured, is not easy.
Accordingly, an object of the present invention is to provide a method of manufacturing a fine T-shaped electrode at low cost and with high throughput without using expensive processes such as electron beam lithography or SOR light exposure.